Sources told the AFP news agency that the Israeli air force had bombed a weapons convoy. However, it was unclear if the attack had taken place on Syrian or Lebanese territory. The Israeli military has refused to comment.

Lebanon's army reported earlier that 12 Israeli warplanes had violated its airspace in the past 24 hours and flown over southern villages.

Israel's vice-prime minister said on Sunday that it might intervene if Syria's military was losing control of its chemical weapons.

Resources exhausted' Addressing the International Humanitarian Pledging Conference for Syria in Kuwait City on Wednesday, Ban Ki-moon describe the situation in Syria as "catastrophic"

MSF said rebels controlled areas where at least a third of the country's population - about seven million people - were currently living.

On Wednesday, the state news agency Sana reported that the families of the victims had "identified a number of the killed, stressing that the al-Nusra Front abducted them because of their refusal to co-operate".

Sana's reporter said the water level in the river was too low to carry the bodies any distance suggesting they had been killed in Bustan al-Qasr.

However, locals said several of those killed were from rebel-held districts and had disappeared after crossing into those under government control. Some had been detained by security forces, they added.

The main opposition alliance, the National Coalition, called for an investigation and warned that "ongoing global inaction towards human rights violations encourages the killers to continue their crimes".